Avian Influenza Viruses

Abstract

Avian influenza viruses comprise all recognised antigenic subtypes within the genus Influenza A of the Orthomyxoviridae family. They are found prominently in waterfowl, shorebirds and terrestrial birds and are sporadically transmitted to mammalian species. The consequences of such transmissions range from a subclinical infection to a mild infectious and/or lethal respiratory disease, as witnessed with highly pathogenic avian H5N1 viruses, which occasionally infect humans (resulting in cases of fatality rates >50%). Further, avian influenza viruses may cause pandemics by transmitting directly to humans (e.g. possibly the 1918 pandemic) or by reassorting with human and/or other (e.g. swine) influenza viruses (1957, 1968 and 2009 pandemics). Avian influenza viruses thus play a critical role in the ecology of influenza A viruses, and in the generation of novel variants that may cause severe infections or pandemic outbreaks in humans.

Key Concepts:

  • Influenza A viruses of all known haemagglutinin (HA) and neuraminidase (NA) subtypes are found in aquatic birds.

  • Avian influenza viruses can be classified as low pathogenic avian influenza (LPAI) or highly pathogenic avian influenza (HPAI), based on their pathogenicity in chickens.

  • All mammalian influenza A viruses originate from avian influenza viruses.

  • Avian influenza viruses play a critical role in the generation of pandemic influenza viruses through direct transmission to humans or reassortment with human and/or swine influenza viruses.

  • Host species‐specific amino acids exist that distinguish avian and mammalian influenza viruses.

  • Host species‐specific amino acids may affect critical functions in the viral life cycle such as receptor recognition and replicative ability; adaptive changes at such key amino acid positions may be critical for avian influenza viruses to cause pandemics in humans.

Keywords: influenza A viruses; HPAI, LPAI; subtype; avian diseases; negative‐strand viruses; haemagglutinin protein; neuraminidase protein, reassortment, pandemic

Figure 1.

Schematic diagram of the influenza viral life cycle. Following receptor‐mediated endocytosis, the viral ribonucleoprotein (vRNP) complexes are released into the cytoplasm and subsequently transported to the nucleus, where replication and transcription take place. mRNAs are exported to the cytoplasm for translation. Early viral proteins, that is, those required for replication and transcription, are transported back to the nucleus. Late in the infection cycle, the M1 and NS2(=NEP) proteins facilitate the nuclear export of newly synthesised vRNPs. PB1‐F2 associates with mitochondria. The assembly and budding of progeny virions occurs at the plasma membrane. Reproduced with permission from Neumann et al., , Nature 459: 931–939.

Figure 2.

Emergence of pandemic influenza viruses. The ‘Spanish influenza’ was most likely caused by the transmission of an avian influenza virus to humans. In 1957, reassortment between an avian H2 and a human H1N1 virus resulted in a virus possessing avian virus H2 HA, N2 NA, and PB1 genes and the remaining genes from the human virus. Similarly, in 1968, reassortment between an avian H3 virus and a human H2N2 virus resulted in a virus possessing avian virus H3 HA and PB1 genes and the remaining genes from the human virus. In 1977, H1N1 viruses reappeared which closely resembled strains that had been circulating in the mid 1950s. The 2009 H1N1 pandemic was caused by reassortment between a triple reassortant swine virus and an Eurasian avian‐like swine virus that contributed the NA and M segments to the novel pandemic virus. Modified with permission from Neumann et al., , Nature 459: 931–939.

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Further Reading

Beato MS and Capua I (2011) Transboundary spread of highly pathogenic avian influenza through poultry commodities and wild birds: a review. Revue scientifique et technique 30: 51–61.

Kido H, Okumura Y, Takahashi E et al. (2011) Role of host cellular proteases in the pathogenesis of influenza and influenza‐induced multiple organ failure. Biochimica et Biophysica Acta [Epub ahead of print].

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Wright PF, Neumann G and Kawaoka Y (2007) Orthomyxoviruses. In: Knipe DM, Howley PM, Griffin DE et al. (eds) Fields Virology, Chapter 48, pp. 1691–1740. Philadelphia: Wolters Kluwer, Lippincott Williams & Wilkins.

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Neumann, Gabriele, and Kawaoka, Yoshihiro(Jul 2012) Avian Influenza Viruses. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001007.pub3]